The present invention pertains to a process for manufacturing films of ultra-high molecular weight polyolefins.
U.S. Pat. No. 5,091,133 describes a method for manufacturing sheets of an ultra-high molecular weight polyolefin by the steps of feeding a polyolefin powder between a combination of endless belts disposed in an up-and-down opposing relation, compression molding the polyolefin powder at a temperature lower than the melting point of the polyolefin powder by means of a pressing device while holding the polyolefin powder between the endless belts, then rolling and stretching the resultant compression-molded polyolefin.
EP 0 467 323 describes a process for the manufacture of coloured films of ultra-high molecular weight polyethylene wherein a dye is added to powdered ultra-high molecular weight polyethylene, which is then subjected to compaction-moulding and stretching.
U.S. Pat. No. 4,879,076 describes a method for manufacturing polyethylene materials by a process comprising compacting and stretching wherein the compacting is carried out in an extruder or in an undefined press.
While the process described in U.S. Pat. No. 5,091,133 above gives a product with acceptable properties, it has been found that there is still room for improvement. In particular, for the manufacture of films with a very high stretching ratio, the process as described in U.S. Pat. No. 5,091,133 may result in products with inhomogeneous quality. An inhomogeneous quality will, int. al., limit the tensile strength of the film.
Accordingly, there is need for a process for the manufacture of films of ultra-high molecular weight polyolefins which results in a product with higher homogeneity, a higher tensile strength, and other desirable physical properties. The process according to the invention also allows the manufacture of wider tapes.